1. Field of the Invention
The present invention is directed generally to a method and an improved system for detecting nitrogenous explosives or narcotics by nuclear quadrupole resonance (NQR), and more specifically, to a lower power method for detecting those materials.
2. Description of the Prior Art
In order to limit the unrestricted flow of explosives and narcotics, it is desired to detect sub-kilogram quantities of those materials in monitoring stations. Most military explosives and narcotics share common features: they are crystalline solids containing nitrogen. Presently, the explosive detections system and methods cannot reliably detect sub-kilogram quantities of military explosives against a background of more benign materials.
In conventional vapor-based systems, dynamites and contaminated explosives may be detected. However, military explosives such as hexhydro-1,3,5-trinitro-s-triazene (commonly referred to as RDX and 2,2-bis[(nitroxy)methyl]-1,3-propanediol, dinitrate (commonly referred to as PETN) are not reliably detected by the conventional vapor base systems especially when countermeasures are taken to reduce the effluent vapor and particles. Thermal neutron systems, which are .sup.14 N detectors, can detect relevant quantities of explosives. Unfortunately, conventional thermal neutron analysis systems frequently alarm on nitrogen-containing plastics. High false alarm rates are produced for inspected bags containing a few bomb equivalents of nitrogen in a benign form since the conventional thermal neutron analysis systems are sensitive only to the nuclear cross sections and not to any details of the particular chemical environment of the detected nitrogen nuclear. Hence the false alarm rate is inherently high, even with some spatial discrimination. Also, nuclear magnetic resonance (NMR) has been considered for detecting explosives. Because a large magnetic field is conventionally required for NMR, magnetically recorded data would be undesirable altered and other magnetizable materials could be damaged. Furthermore, the conventional non-vapor methods and systems are not suitable for inspecting people.
The parent application of the present invention, Buess et al., DETECTION OF EXPLOSIVES BY NUCLEAR QUADRUPOLE RESONANCE, Ser. No. 07/704,744, Navy Case No. 72,625, filed May 23, 1991 (the entirety of which is incorporated herein by reference), discloses a method and system for NQR detection of explosives. Recited advantages of NQR for explosives detection are:
(i) Specificity: the NQR resonant frequency of a quadrupolar nucleus in a crystalline solid is quite well-defined. Most explosives of interest contain nitrogen and are crystalline solids. Most nitrogen found in the contents of airline bags is in a polymeric form, with associated broad, weaker NQR resonances and generally at frequencies other than the characteristic frequencies of the explosive. NQR is sensitive to the chemical structure, rather than just the nuclear cross-section, as in the thermal neutron analysis approaches. For NQR, false alarms from other nitrogenous materials will be far less of a problem than in nuclear-based detection techniques. PA0 (ii) Sensitivity: though NQR is not a very sensitive spectroscopy, the parent disclosure describes techniques to make the response more sensitive to the desired explosive and less sensitive to interfering signals. Sensitivity is a function of coil geometry and coil size. The invention described in the parent disclosure has demonstrated sensitivity to detect the equivalent of sub-kilogram quantities of explosive near a brief case-sized meanderline coil and substantially less explosives in a small solenoidal coil of 25 mm diameter in a few seconds. PA0 (iii) Localization: one of the novel features of the NRL approach is to localize the transmitting field and the receiver by a specialized surface coil, never previously used for NQR. One type of surface coil, the `meanderline` coil, localizes the sensitive inspection region to a well-defined region. Furthermore, the electrical and magnetic fields of the meanderline coil fall off very rapidly with distance, so that a person can be scanned by an NQR detector without depositing substantial rf power into the body.